Brush the cooking grates clean. Grill the steaks over direct high heat, with the lid closed, until cooked to your desired doneness, 8 to 10 minutes for medium rare, turning once or twice. Remove from the grill and let rest for 3 to 5 minutes.

Spread the mushrooms in a single layer on the grill pan and cook over direct medium heat, with the lid closed, until golden brown and tender, 6 to 8 minutes, turning once or twice. Don’t move the mushrooms until the bottom sides are nicely browned. Transfer the mushrooms to the skillet with the bacon and onions and warm over medium heat for about 1 minute, stirring often. Add the chives and mix well. Serve the steaks warm with the mushroom mixture spooned on top. If desired, whisk the reserved vinaigrette and drizzle some over each steak.